Andrew Gallup, a postdoctoral research associate at Princeton and his co-author Omar Eldakar, a postdoctoral fellow in the University of Arizona's Center for Insect Science conducted the study.

For their research, the study authors measured how frequently 160 people yawned outdoors in the winter, compared to summer. Eighty people were observed for each session in Tucson, Arizona.

In the summer, people were less likely to yawn. But in the winter, when temperatures were cooler outside, the researchers found people yawn more.

Researchers have never reached a consensus as to why people yawn, though dozens of scientists have theories.

Gallup says "Enter the brain cooling, or thermoregulatory, hypothesis, which proposes that yawning is triggered by increases in brain temperature, and that the physiological consequences of a yawn act to promote brain cooling.

"According to the brain cooling hypothesis, it is the temperature of the ambient air that gives a yawn its utility. Thus yawning should be counterproductive -- and therefore suppressed -- in ambient temperatures at or exceeding body temperature because taking a deep inhalation of air would not promote cooling. In other words, there should be a 'thermal window' or a relatively narrow range of ambient temperatures in which to expect highest rates of yawning.”

The researchers concluded yawning doesn’t relieve an overheated brain when ambient temperatures are higher than or equal to body temperature.

Yawning might increase blood flow to the brain when the jaw stretches. Inhaling cool air produces a "countercurrent heat exchange that cools the brain", explain the researchers.

During the study, outdoor temperatures in the summer averaged 98.6 degrees F, with low humidity. In the winter, temperatures were 71 degrees with slightly higher humidity, according to Gallup.

The participants were chosen at random – the researchers recruited pedestrians, who were also asked to view “contagious” yawning photographs as part of the research.

Less than a quarter of people yawned in the summer, compared to half of people in the winter.

The study authors also found yawning frequency tapered off with more time spent outdoors in the summer.

“Nearly 40 percent of participants yawned within the first five minutes outside, but the percentage of summertime yawners dropped to less than 10 percent thereafter”, says Gallup. “An inverse effect was observed in the winter, but the proportion of people who yawned increased only slightly for those who spent more than five minutes outdoors.”

According to the authors, the finding may lead to more understanding of diseases and conditions that upset the thermoregulatory mechanism in the brain. Multiple sclerosis and epilepsy are accompanied by frequent yawning.

The Princeton research sheds more light on why people yawn. Frequent yawning might be the brain’s way of staying cool.